The White House promised a comprehensive series of measures, including gun control legislation, on Monday to prevent a recurrence of mass shootings such as the "horrific" attack in Connecticut that left 20 children and six teachers dead.

The pledge came as the first cracks began to appear in the seemingly impregnable opposition to gun controls. Less than 24 hours after Barack Obama signalled that gun control would be a second term priority during a powerful speech on Sunday evening at a vigil for the victims in Newtown, at least two Democratic senators strongly identified with gun rights shifted position in favour of reform.

The killings in Connecticut appear to be bringing about a change in mood that was not evident after shooting sprees over the last decade. They include Virginia Tech in 2007, the attack on congresswoman Gabrielle Giffords last year and the Aurora cinema shootings in July this year.

The White House spokesman, Jay Carney, at the daily briefing went further than Obama the night before in elaborating on what kind of measures might be taken. At the emotional vigil in Newtown, Obama appeared to abandon his reluctance to take on the gun lobby and delivered an impassioned speech in which he said change had to come.

Addressing an audience of about 900 local people, including about 15 families of victims, at Newtown high school he talked about the disaster at Sandy Hook elementary school, in which "20 beautiful children and six remarkable adults" had died.

Asking if the country had fulfilled its obligation to protect its children, he said: "The answer is no."

He added: "In the coming weeks I'll use whatever power this office holds to engage my fellow citizens from law enforcement, mental health professionals to parents and educators in an effort aimed at preventing more tragedies like this."

He did not specify what change he had in mind, or even whether it would include new legislation on gun control.

But Carney, when asked what kind of measures the president had in mind, said: "It is a complex problem that will require a complex solution. No single piece of legislation, no single action will fully address the problem."

Proposals that have been put forward since Friday range from the renewal of a lapsed ban on automatic and semi-automatic weapons and restricting the number of bullets in a clip. Other proposals suggest addressing how communities deal with mental health issues.

Asked if gun control legislation would be part of this comprehensive package, Carney said it would be.

Carney pointed out that Obama had long been an advocate of "common sense measures". Before his election in 2008, Obama had supported renewal of a 1994 ban on automatic weapons that lapsed in 2004.

Asked whether the president would move to exploit the present mood or wait until he had dealt with issues such as the fiscal cliff standoff and immigration reform, Carney suggested the former, reminding reporters that Obama had spoken in terms of "weeks".

The hopes of gun reform advocates rose when two Democratic senators, Joe Manchin and Mark Warner, came out in favour of reform. Both had been staunchly in favour of gun rights.

Manchin, in particular, was a surprise, given that he uses a picture of himself holding a rifle in his campaign literature, is backed by the National Rifle Association and, to publicise his opposition to trade legislation, once fired a shot through a copy of the bill.

Manchin, who represents West Virginia, told MSNBC's Morning Joe show that the Connecticut shooting had changed the dialogue.

"It's time to move beyond rhetoric. We need to sit down and have a common sense discussion and move in a reasonable way. This has changed the dialogue and it should move beyond dialogue – we need action," Manchin said.

He argued it was possible to protect gun ownership for hunting while banning other automatic weapons and large ammunition clips.

"I don't know anyone in the sporting or hunting arena that goes out with an assault rifle," Manchin said. "I don't know anyone that needs 30 rounds to go hunting. I mean these are things that need to be talked about."

Warner, in an interview with CBS, expressed continued support for the constitutional right to bear arms as enshrined in the second amendment.

"I believe every American has second amendment rights; the ability to hunt is part of our culture. I've had a NRA rating of an 'A' but, you know, enough is enough," Warner said. "I think most of us realise that there are ways to get to rational gun control. There are ways to grapple with the obvious challenges of mental illness."

While opposition to gun controls is strongest in the Republican party, part of the reason for the failure to get legislation through recently has been opposition from many Democrats fearful of upsetting the gun lobby.

The Democratic leader in the Senate, Harry Reid, normally an opponent of attempts to reform gun laws, including a no vote in 2004 to extend the ban on automatic weapons, also shifted ground. Speaking on the floor of the chamber, he said every idea must be on the table. "We need to accept the reality that we are not doing enough to protect our citizens," he said.

His normal opposition to attempts to introduce gun regulations had won him the backing of the NRA up until this year, when it did not endorse him in the November Senate race.

The first obstacle to getting legislation through is winning over Democrats such as Reid, Warner and Manchin. The next stage is winning over at least some Republican support.

Republicans have largely been lying low. The Guardian approached 28 strongly pro-gun Republican senators, all of them with 'A' grades from the NRA, and most declined to respond.

Republican senator Richard Shelby, normally a hardline supporter of gun rights, did not hint at any shift but did at least express a polite willingness to engage. A spokesman for the senator from Alabama said: "He believes we must consider any proposals very carefully."

Another Republican senator, Johnny Isakson, went further, expressing support for a broad inquiry into the whole issue.

"After 9/11, we came together to see what we could do to prevent another attack. The work of the 9/11 commission made flying safer and has helped us prevent another hijacking of a US plane by terrorists. We should evaluate in a thoughtful process gun safety, mental health, school security and all other components that contributed to this horrendous act of violence," Isakson said in a statement to the Guardian.

The Democratic senator Dianne Feinstein, who led the push for the ban on automatic weapons in 1994, said on Sunday she will introduce a similar bill in January next year that will ban automatic and semi-automatic weapons and the sale of clips containing multiple bullets.

Another Democratic congresswoman, Carolyn McCarthy, whose husband was killed in a shooting spree, called for background checks on all gun sales, in particular at gun shows which are not subjected to such checks.

A new poll in the Washington Post/ABC showed a shift in attitudes, with a majority, albeit slim, now viewing the shooting not as an isolated act but part of society's wider problems.

But there is little change in views towards gun control. While a majority favour a ban on ammunition clips containing a large number of bullets, the poll shows support for gun ownership remains ingrained in America, with 71% opposed to a ban on the sale of handguns.